refactor: Remove NEXTCLOUD_OIDC_CLIENT_STORAGE environment variable

Remove the NEXTCLOUD_OIDC_CLIENT_STORAGE environment variable from all
configuration files. OAuth client credentials are now always stored in the
SQLite database, with no option to use a custom JSON file path.

Changes:
- Remove NEXTCLOUD_OIDC_CLIENT_STORAGE from .env.keycloak.sample
- Remove NEXTCLOUD_OIDC_CLIENT_STORAGE from docker-compose.yml (mcp-oauth and mcp-keycloak services)
- Remove NEXTCLOUD_OIDC_CLIENT_STORAGE from Helm deployment template
- Remove NEXTCLOUD_OIDC_CLIENT_STORAGE from test_cli.py test assertions
- Remove --headed flag from pytest addopts (use CLI arg instead)

This simplifies configuration by enforcing a single storage mechanism
(SQLite database) for OAuth client credentials.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

charts/nextcloud-mcp-server/templates/deployment.yaml

@@ -78,8 +78,6 @@ spec:
               value: {{ include "nextcloud-mcp-server.mcpServerUrl" . | quote }}
             - name: NEXTCLOUD_PUBLIC_ISSUER_URL
               value: {{ include "nextcloud-mcp-server.publicIssuerUrl" . | quote }}
-            - name: NEXTCLOUD_OIDC_CLIENT_STORAGE
-              value: "/app/.oauth/nextcloud_oauth_client.json"
             - name: NEXTCLOUD_OIDC_SCOPES
               value: {{ .Values.auth.oauth.scopes | quote }}
             {{- if .Values.auth.oauth.clientId }}